> When I try to mirror few websites into the same directory.
> For example i create /somedir/mirror/ and I want to use it
> this way:
> httrack <http://site1.com/> -O /somedir/mirror/ 
> and then 
> httrack <http://site2.com/> -O /somedir/mirror/
> I get errors 'bout cache files and index.html file.

You can't do that - you can mirror only one *project* per 
directory ; that is:
httrack <http://site1.com/> -O /somedir/mirror/site1
httrack <http://site2.com/> -O /somedir/mirror/site2

If you want to have these two sites "merged" together, 
mirror them in one shot:
httrack <http://site1.com/> <http://site2.com/> -
O /somedir/mirror/mysite